This application is based on Patent application No. 2000-186951 filed Jun. 21, 2000 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a printing apparatus applied to copying machines, printers, word processors and facsimiles and more specifically to a printing apparatus with a print head capable of ejecting ink droplets.
2. Description of the Related Art
Printing apparatuses with functions of a printer, copying machine and facsimile machine and printing apparatuses used as output devices for combination type electronic apparatuses including computers and word processors and for work stations are all designed to record an image on printing media, such as paper and plastic thin plates, according to image data. Such printing apparatuses can be classed into an ink jet type, wire dot type, thermal type and laser type according to the printing method. The ink jet type printing apparatus (ink jet printing apparatus) ejects ink from nozzles of an ink ejection means such as a print head onto a print medium to form an image and has a variety of advantages.
They include abilities to reduce the size of the ink ejection means, print a very fine image at high speed and print on plain paper without requiring a special treatment on the paper, a low running cost, low noise because of the non-impact printing method, and an ease with which a color image can be formed using multicolor inks. Particularly in a print head using a so-called bubble jet printing method that utilizes thermal energy in ejecting ink, liquid passages can easily be arranged at high densities (high density nozzle arrangement) by using a semiconductor manufacturing process including etching, vapor deposition and sputtering. This in turn leads to a further reduction in size.
Even in this ink jet printer the ink droplets are not necessarily ejected in a constant direction at all times. That is, the ink ejection direction may vary from one nozzle to another depending on wet conditions of the ink ejection ports, conditions of paper dust adhering to the ink ejection ports and shape differences among the ink ejection ports.
During printing at a maximum density, i.e., in a 100% duty printing condition, any error (deviation) in the ink ejection direction easily shows up. This ejection direction deviation results in blank lines appearing in a solid-printed image, i.e., a color of the print medium itself showing through. Hence, the image formed with ink is so designed that the above-described problem of blank lines on a printed image does not occur if the ink ejection direction deviation is within a predetermined value. One of such blank line prevention means increases an ink penetration area on the paper so that slight ejection direction deviations will not produce blank lines.
Even with such means provided, in the event that ink ejection direction deviations exceed the ink penetration area, image failures called blank lines may result. Identifying the nozzles that cause such ejection direction deviations is as important as taking measures for correcting the ejection direction deviations of these nozzles. The identifying of the faulty ejection nozzles is becoming harder than ever because the resolution of the printed image in recent years is as high as 600 dpi to 1440 dpi and the area in which an ink droplet can seep after landing on the print medium is becoming increasingly smaller. In a printing apparatus employing binary representation, the miniaturization of the ink droplet (print dot) is particularly important in terms of eliminating the granular appearance of ink dots and enhancing an image quality The ink droplet miniaturization, however, makes the identifying of faulty ink ejection nozzles more difficult.
A conventional method of detecting a deviation of ink ejection direction involves setting constant a distance between a print medium and ink ejection nozzles (simply referred to as nozzles), ejecting ink from the nozzles onto the print medium to print an image, measuring a distance from a predetermined reference ejection position to a position of a resulting blank line, and identifying a deviated nozzle according to the measured distance.
With the conventional nozzle identification method, however, because the print medium and the nozzles are set to have a distance or positional relation that optimizes the printed image quality, i.e., the positional relation that minimizes the deviation, an apparatus for measuring the deviated position needs to be a high-performance image reading device. This is because an increased resolution makes the ink seeping area forming a printed dot smaller and the measurement of density variations among the printed dots requires the use of a measuring device with a resolution several times higher than that of the printed dots.
The present invention has been accomplished to overcome the problems of the conventional technique described above. It is an object of the invention to provide a printing apparatus which has an ink ejection means capable of forming an image at high resolution and still can reliably identify a location where an ink ejection error has occurred without using an expensive image reading device.
To solve the problems above, the present invention has the following construction.
According to a first aspect, the invention provides a printing apparatus for ejecting ink droplets from an ink ejection means onto a print medium to form an image, the printing apparatus comprising: a distance adjusting means to change a distance between the ink ejection means and the print medium; and a control means to set a plurality of different distances by controlling the distance adjusting means; wherein the control means controls the distance adjusting means to selectively set either a first distance that produces a normal printed image or a second distance.
In the invention above, the first distance may be a distance that produces a normal printed image having within a predetermined range a landing error (deviation from a landing position) on the print medium of ink droplets ejected from the print head, and the second distance may be a distance that produces a landing error greater than the predetermined range.
In the invention above, the first distance may be a distance that produces a normal printed image and the second distance may serve as a print position for producing a printed image and as a distance for observing a landing error of the ink droplets.
According to another aspect, the invention provides a printing apparatus for ejecting ink droplets from an ink ejection means onto a print medium to form an image, the printing apparatus comprising; a distance adjusting means to change a distance between the ink ejection means and the print medium; and a control means to set a plurality of different distances by controlling the distance adjusting means; wherein the control means controls the distance adjusting means to selectively set either a plurality of first distances that produce a normal printed image having within a predetermined range a landing error on the print medium of ink droplets ejected from the print head or a plurality of second distances that produce a landing error greater than the predetermined range.
In the invention above, the first distance may be a distance that produces a normal printed image and the second distance may serve as a print position for producing a second printed image and as a distance for observing a landing error of the ink droplets.
Further, in the invention above, it is desired that the first distance be a distance such that adjacent ink droplets join through a penetration of ink as a result of the landing error and that the second distance be a distance such that an unpenetrated ink area is formed between the adjacent ink droplets as a result of the landing error.
In the invention above, when the ink droplet ejection error characteristic of the ink ejection means is to be measured, a measurement command is entered into the control means. In response to this command, the control means controls the distance adjusting means to shift the distance between the print medium and the ink ejection means from the distance position that produces a normal printed image. Performing the printing operation under this condition results in a large landing error of ink droplets due to the ink ejection direction deviation. This causes a significant image degradation and thus allows the user to identify a portion of the ink ejection means that has caused the ejection error. When the user wishes to form a normal printed image, the user enters a predetermined print command from an input device. As a result, the control means controls the distance adjusting means to form an appropriate printed image that has no large landing errors.